Sift the flour into a medium mixing bowl. Using two knives, cut in the butter until it is the size of small peas. Stir in the sugar. Knead together all the ingredients and press the dough evenly into the cake pan. Bake for 25 minutes, or until the shortbread becomes a cream color. Watch to ensure that the shortbread does not brown. Remove from oven and let cool in the pan.

For the toffee filling:

Combine the first five ingredients in a medium saucepan. Over medium-low to medium heat, stir constantly until the mixture comes to a boil. Boil for seven minutes, continuing to stir so the caramel won’t burn. Remove from the heat, completely stir in the vanilla extract, and pour the filling onto the shortbread base. Cool to room temperature, then refrigerate for 15 minutes.

For the topping:

Gently melt the chocolate on low in a microwave or double boiler. Spread it over the toffee filling. Refrigerate the pan until the toffee hardens.

Run a knife around the inside edge of the baking pan. Invert the pan onto a dry cutting board and press the pan to release the dessert. Turn the dessert over and cut it into one-inch squares.